Evaporators of the above mentioned general type are typically embodied as plate-type or tubular heat exchangers. Such heat exchangers are typically used in applications in which a medium is stored in liquid form at a low or super-cold temperature in a storage tank, but is to be provided for use in a gaseous physical phase. An example of such an application is the use of cryogenic liquids, such as liquid hydrogen or liquid natural gas, as a fuel or energy carrying medium for aircraft propulsion engines, and especially turbine engines.
A particular feature relating to such evaporator devices is that the liquid medium to be evaporated is delivered to the evaporator with a very low inlet temperature of only about 20 K (-253.degree. C.). On the other hand, a relatively hot medium, such as the surrounding atmospheric air, or the exhaust gas of an engine for example, or some other heated medium, is provided to the evaporator as a heat source for evaporating and in some cases superheating the cryogenic liquid medium. Thus, the hot medium comes into contact with extremely cold surfaces of the evaporator, which are cooled by the cryogenic medium that is to be evaporated. As a result there is a danger that the hot medium will be cooled to below its respective dew point temperature or freezing point temperature so that it at least partially condenses or forms ice on the surfaces of the evaporator. Such condensation or ice build-up obstructs the flow passages for the hot medium, interferes with the operation of the evaporator, and can present a serious danger, especially if the evaporator is part of a fuel preparation system for an aircraft engine.
Typically, this danger of freezing or condensing of the hot medium is avoided or counter-acted by reducing the heat transfer between the hot medium and the cryogenic medium that is to be evaporated. In this manner, higher surface temperatures can be achieved at the heat inlet side of the evaporator. However, this in turn necessitates a larger and heavier structure of such an evaporator in order to achieve the same total or overall heat transfer and evaporation of the cryogenic medium, which is especially undesirable in the application of such evaporators in aircraft and spacecraft.